Difflugia lanceolata Penard, 1890
Diagnosis: Shell clavate, elongate and composed of more or less flat siliceous angular particles arranged so that the sides appear polished and smooth. Fundus shaped like an ovigals arch. Aperture circular and often surrounded by an organic collar. Nucleus ovular, with small roundish nucleoli, some of them flattened against the nucleolar membrane. No zoochlorellae.
Penard (1902) described this species as follows: “The shell is always lanceolate, uncompressed, almost smooth, very regular in its contours; the shape is roughly that of a more or less elongated oak acorn. The fundus is almost always regularly rounded; more rarely there is a tendency to an ogival shape, more or less pronounced, or even, in very exceptional cases, to the formation of a terminal nipple. The shell is hyaline, thin, chitinoid, and always covered with flat, thin siliceous particles. Rarely, a few small stones are also noticed on the surface. Plasma is abundant, and always contains many small, colorless shiny grains, then, constantly also, pale starch grains, very small, not spherical, but elongated and curved. Zoochlorellae never present. The nucleus is spherical; its diameter is 28 µm on average; it is very pale, very beautiful, with a fine and frank membrane. The interior is filled with a nuclear juice of a light greenish blue, in which can be seen, especially on the surface, pale and delicate nucleoli, flattened, discoid, with tendency to a crescent shape. Between these nucleoli are embedded pale, rounded grains, much smaller, like dust. Pseudopods are large and few in number: most often only one or two are seen. The size generally varies between 140 and 160 µm.”
Dimensions: Penard (1902): Length 140-160 µm; nucleus ≈ 28 µm. My measurements 138-200 µm; nucleus 23-26 µm (n=3).
Ecology and distribution: Freshwater, several water types. I found it also in eutrophic water, in ditches between heavily fertilized farmland. Chardez (1978) found that “D. lanceolata and D. penardi are species which are sometimes found in limnosaprobic muds (…)”.
Remarks: Sometimes the fundus is a little asymmetrically pointed, but the clean outline of this species is the main distinguishing feature, though also specimens with rougher xenosomes are found.
Remarks: In the spring of 2012 I had also an interesting sample from a small floating piece of debris, coming from the bottom of a shallow ditch along the Naardermeer. The sample contained a huge collection of shells, most empty, but the smaller ones were inhabited. While all shells were present in some cubic centimeters of debris, it is likely that they were all related.
I measured the length of 469 shells. The results are given in the graph below. It is remarkable that there are at least two populations. One possibility is that there are smaller shells in spring and larger shells later in the year. That could explain that in my sample (spring) only smaller shells were inhabited. A second explanation could be that there are at least two different species, but these cannot be distinguished light microscopically.